Hi Mamu!
I still have my tag from the "Blackie" sweet potato vine I bought last year, and it says it has a spread of 12". I had one in a 12" pot with other annuals, and it probably had about an 18" spread. Are you putting anything else in the hanging pot? If not, I'd probably put 4 in, you can always remove one if it gets too crowded.!
It's always a gamble..........
Word of caution!! This is a quote from the Sweet Potato Vine link I put in the "Favourite Container Annuals" thread.
"This vine is a vigorous grower. In the spring I started with some 4 inch pots of vines and in just a few weeks they grew into 12 foot plants. Now that's what I call vigorous growth."
I can't imagine what they must of been feeding it!!!!!!!!
